US 8014263 B2 Abstract Cross-talk is canceled in a cooperative wireless relay network that includes a base station (BS), a relay station (RS), and a mobile station (MS). A coupling channel between a transmit antenna and a receive antenna colocated at the RS is estimated. Cross-talk interference determination is based on a previous transmitted signal by the transmit antenna, and the coupling channel. The cross-talk interference is subtracted from a currently received signal by the receive antenna to obtain a residual signal. The residual signal is then transmitted as a next transmitted signal by the transmit antenna.
Claims(14) 1. A method for canceling cross-talk in a cooperative wireless relay network including a base station (BS), a relay station (RS), and a mobile station (MS), comprising a processor for performing the steps, comprising the steps:
estimating a coupling channel between a transmit antenna and a receive antenna, wherein the transmit antenna and the receive antenna are colocated at the RS;
determining cross-talk interference based on a previous transmitted signal by the transmit antenna, and the coupling channel;
subtracting the cross-talk interference from a currently received signal by the receive antenna to obtain a residual signal, wherein for K-subcarrier orthogonal frequency-division multiplexing (OFDM), the received signal is represented by OFDM modulation, the received symbol vector over the k
^{th }subcarrier of the n^{th }OFDM symbol is y(n,k)=s(n,k)+i(n,k)+w(n,k), where n represents an OFDM symbol, k represents a subcarrier, s represents a desired signal, i represents the cross-talk interference, and w represents noise; andtransmitting the residual signal as a next transmitted signal by the transmit antenna.
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14. A relay station in a cooperative wireless relay network including a base station (BS) and a mobile station (MS), comprising:
means for estimating a coupling channel between a transmit antenna and a receive antenna, wherein the transmit antenna and the receive antenna are colocated at the RS;
means for determining a cross-talk interference based on a previous transmitted signal by the transmit antenna and the coupling channel;
means for subtracting the cross-talk interference from a currently received signal by the receive antenna to obtain a residual signal, wherein for K-subcarrier orthogonal frequency-division multiplexing (OFDM), the received signal is represented by OFDM modulation, the received symbol vector over the k
^{th }subcarrier of the n^{th }OFDM symbol is y(n,k)=s(n,k)+i(n,k)+w(n,k), where n represents an OFDM symbol, k represents a subcarrier, s represents a desired signal, i represents the cross-talk interference, and w represents noise; andmeans for transmitting the residual signal as a next transmitted signal by the transmit antenna.
Description This invention relates generally to wireless relay networks, and more particularly to canceling cross-talk in cooperative wireless relay stations. In wireless networks, a base station (BS) is usually located near the center of a cell. As a result, the quality of signals received at a mobile station (MS) near an edge of the cell is reduced. This problem can be resolved by decreasing the size of the cell. However, this increases costs because the number of BSs needs to be increased to provide the same service for the same coverage area. In addition, this also may cause higher level of interference to MSs residing at the edge of neighboring cells. An alternative solution uses a less complex relay station (RS), which assists in communications between the BS and the MS. The RS can effectively increase coverage and reliability with decreased transmit power and cost. The RS can be deployed anywhere in the cell wherein direct communication between the BS and the MS is impaired. The RS can also be deployed temporarily in areas where the number of MSs is expected to increase dramatically for a short time period, e.g., large-scale public events. The RS can use decode-and-forward (DF) or amplify-and-forward (AF) modes. The DF mode detects and demodulates received signals before retransmitting. The AF mode only amplifies the received signal before retransmitting. Conventionally, the RS can use different frequencies or times to reduce interference. In a frequency-reuse-relay-station (FRRS), the frequency bands for transmitting and receiving signals are identical. In a frequency-shifted-relay-station (FSRS), the frequency bands are different. While the FRRS increases spectral efficiency, cross-talk interference becomes an issue because the transmit signal power is always greater than the receive signal power. In the FSRS, out-of-band leakage can cause cross-talk interference. Cross-talk occurs when a transmitted signal interferes with a currently received signal. Cross-talk can be caused by undesired capacitive, inductive, or conductive coupling between the transmit antenna and the colocated receive antenna, or lines and circuits to which the antennas are connected. Cross-talk is often denoted as co-channel interference, and is related to adjacent-channel interference, see Nasr et al., “Performance of an echo canceller and channel estimator for on-channel repeaters in DVB-T/H networks,” IEEE Trans. Broadcasting, vol. 53, no. 3, pp. 609-618, September 2007, and Mazzotti et al., “Performance of an echo canceller based on pseudo-noise training sequences,” Proc. 58th Annual IEEE Broadcast Symposium, October 2008. Prior art techniques require the RS to transmit dedicated pilot signals, such as pseudo-noise sequences, for estimating the coupling channel between the colocated transmit and receive antennas. The pilot signals change the existing signal structure of the physical layer, leading to incompatibility with legacy standards, and also result in interference at the receiver. It is desired to perform cross-talk cancellation at the RS that is transparent to current wireless standard, i.e., the structure of the signals at physical layer remains the same at the BS and the MS. The embodiments of the invention object provide a method for canceling cross-talk interference in a wireless frequency-reuse-relay-station (FRRS) or a frequency-shifted-station (FSRS) using signals that are designed according to current wireless standards. The RS estimates a coupling channel between a transmit antenna and a colocated receive antenna from a previous transmitted signal and a currently received signal. Based on the coupling channel, the cross-talk interference at the RS can be reconstructed, and cross-talk interference can be reduced. Relay Network The RS operates as a frequency-reuse-relay-station (FRRS), or as a frequency-shifted-relay-station (FSRS). The RS can cooperate by using the identical frequency band for receiving and transmitting. Thus, depending on the specific cooperative scheme applied, there can be direct communication links Cross-Talk Interference at the RS The RS can have multiple transmit antennas colocated with multiple receive antennas. A desired received symbol vector at the RS is s(n) in the n Cross-Talk Cancellation Based on Coupling Channel Estimation We estimate To cancel Because the previous transmitted signal {circumflex over (x)}(n−1) Cross-Talk with OFDM Modulation In one embodiment of our invention, we cancel cross-talk at the RS with a set M A multiple-tap, time-domain coupling channel has a maximum delay of L OFDM sampling intervals, with the M _{M}, denotes the M_{r}×M_{r }identity matrix, vec(H_{c,l}) denotes the vectorization of H_{c,l }formed by stacking the columns of H_{c,l }into a single column vector.
We define We determine the transmitted symbol matrix at the RS in the (n−1)
Therefore, the original received symbol vector of the RS in the n We denote the composite transmitted symbol matrix of the RS in the previous N OFDM symbols as
Cross-Talk Cancellation Based on Least-Square Coupling Channel Estimation Because the previous transmitted symbol matrix, {circumflex over (X)} Based on the estimated coupling channel ĥ Our cross-talk cancellation method based on the LS estimation of the coupling channel is applicable to the RS with decode-and-forward (DF) or amplify-and-forward (AF) modes, or other relay mechanism. Furthermore, the method is applicable to both an FRRS and an FSRS. When applied to an FRRS, {circumflex over (X)}(n−1) denotes the transmitted symbol matrix at the RS in the (n−1) Coupling Channel Estimation and Cross-Talk Cancellation at a DF-Based RS In one embodiment of our invention, the source station (SS) has M, (≧1) transmit antennas, which, depending on the direction of communication can be the BS or the MS. If we denote the M If we denote s(n)=(s(n,0) In the case that the RS uses the DF mode, both the transmitted symbols of the SS and those of the RS have identical power, and the wireless channel from the SS to the RS is subject to Rayleigh fading. It can be shown that the composite signal matrix in the previous N OFDM symbols s In our invention, R In the case that the transmitted signals of the SS are independent across different OFDM subcarriers and different OFDM symbols, the multi-tap time-domain channel between the SS and the RS has independent channel gains over different taps, and the M When the correlation matrix of the composite coupling channel vector R The corresponding correlation matrix of the residual error vector after cross-talk cancellation is Similarly to LS coupling channel estimation, in the case that R Based on the above simplification, the MMSE coupling channel estimation is performed in our invention as shown in Simulation Results We simulate a cooperative network in which the BS, the wireless FRRS, and the MS each have one transmit antenna and one receive antenna. OFDM modulation with 16 subcarriers is utilized for broadband transmission in this wireless network and QPSK modulation is applied over each OFDM subcarrier. The maximum delay of the multiple-tap time-domain coupling channel from the transmit antenna to the receive antenna of the RS is two OFDM sampling intervals. The channel gains over the two taps are with independent Rayleigh fading, while the average powers decay with the delay exponentially with the exponent factor one. The wireless channel between the source station, which may be the BS or the MS, and the RS is also with Rayleigh fading. Furthermore, the signal-to-noise ratio (SNR) at the RS is set to be 40 dB. In the simulation, the signal-to-interference ratio (SIR) is defined as the ratio of the average desired signal power to the average cross-talk interference power before cross-talk cancellation; the normalized mean-square error (MSE) after cross-talk cancellation is defined as the average power of the residual error normalized to the average power of the desired signal over each OFDM subcarrier. The Figure shows that when the SIR is less than 0 dB, which is the usual case in practice, the cross-talk cancellation based on the LS and the MMSE coupling channel estimation achieve a similar improvement gain. When the SIR is greater than 0 dB, the MMSE coupling channel estimation achieves a higher improvement gain than the LS coupling channel estimation. When the SIR is higher than 15 dB, the cross-talk cancellation based on the MMSE coupling channel estimation still achieves an improvement gain, while the estimation based on the LS coupling channel cannot. Patent Citations
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